This is six in on hand and a half a dozen in the other. Which side makes you feel more comfortable?

Howard has a better offensive line than Fournette.
Howard faces more "soft" defenses than Fournette. Fournette starts the season against Houston, Tennessee, Baltimore, the Jets, Pittsburgh and the Rams. I think they'll all be tough against the run. Then he'll have another game with Houston and Tennessee. And he gets Seattle later in the season. Howard's schedule won't be easy. But he will have New Orleans, Green Bay, Detroit, Philly and San Fran in the middle of the season. And he gets two games against Green Bay and Detroit.
And Fournette is starting the season with a foot injury.

Pryor had a breakout season last year. But he is on a new team with a new quarterback. It may take time to establish chemistry with Cousins.
A.J. Green and Andy Dalton have established chemistry. But Green has had to miss time with injuries in two of the last three seasons.

i'm conservative. i would choose Howard/Green and try to find another receiver just in case Green is injured again.

But if Fournette is healthy and the line excels and Pryor clicks with Cousins, they will have a high upside.